1. (Field of the Invention)
The present invention relates mainly to a construction machine having an upper rotating structure. Particularly, the invention is concerned with a control circuit in a hydraulic excavator.
2. (Description of the Related Art)
FIG. 10 is a circuit diagram of a rotating motor described in Japanese Utility Model Laid Open No. 41203/91.
In the rotating circuit of the prior art shown in FIG. 10, when a rotating hydraulic remote control valve 2 for starting the operation of a rotating motor 1 is operated, a secondary pilot pressure derived from the valve 2 acts on pressure increasing ports 4L and 4R of relief valves 3L and 3R, respectively, whereby the relief valves 3L and 3R are set at a high relief pressure. As a result, the starting torque of the rotating motor 1 is increased and a satisfactory accelerating performance can be exhibited at the time of start-up of rotation.
For stopping the rotation of an upper rotating structure, the rotating hydraulic remote control valve 2 is returned to its neutral position, whereby the secondary pilot pressure is no longer exerted on the pressure increasing ports 4L and 4R, so that the set pressure for the relief valves 3L and 3R shifts to a lower pressure side. Thus, a braking force working on the rotating motor 1 does not act abruptly. In other words, since the braking force for the motor 1 copes flexibly with the force of inertia of the upper rotating structure, there is performed a smooth stop of the upper rotating structure.
FIG. 11 is a side view showing the posture of a working attachment 7 attached to an upper rotating structure 6 of an hydraulic excavator 5.
The working attachment 7 comprises a boom 8, an arm 9, and a bucket 10 as a working tool, which are connected successively with one another. As shown in phantom in FIG. 11, when the boom 8 is brought down and the arm 9 extended, a working radius R from a rotation center O--O of the upper rotating structure 6 to the front end portion of the arm 9 is long. In this state, an inertial mass of the upper rotating structure 6 is large. A rotation starting force of a rotating motor 11 is set so as to permit start-up of rotation even in the case of a large inertial mass. On the other hand, when the boom 8 is raised and the arm 9 retracted, as indicated with solid line in FIG. 11, a working radius, r, from the rotation center O--O of the upper rotating structure 6 to the front end portion of the arm 9 is short, and the inertial mass of the upper rotating structure 6 is small. Particularly, in the case of a construction machine with a small rotational inertia such as, for example, an intra-width swing machine or a small-size excavator, the operability for rotation varies markedly according to the posture of the working attachment. In a reduced state of the working radius of the working attachment, a change in rotational force, which occurs in response to the operation of a lever for actuating a hydraulic cylinder for the boom or for the arm, is oversensitive and therefore operation is difficult. This problem occurs also in a rotating operation with a small working load imposed on the boom or in a combined operation for both traveling and rotation. Anyhow, it cannot be said that the operator is given a comfortable feeling in the rotating operation.